The well was drilled with sea water and the formation is soft clay formation. Then, the bit was totally plugged. We did not have any chance to displace well with water based mud prior to coming out of the hole. We flow checked and came out of hole to change the bit.
Because the formation is clay which will react with seawater resulting formation swelling, we need to develop specific rig procedure to deal with when tripping in hole.
What are the foreseeing issues?
• Clay formation swell
• Pack off due to formation instability
• Soft formation tends to get side tracked easily with mud motor
What is the objective?
• Attempt to condition the well before running back in hole with motor assembly to minimize chance of side track.
These are out rig specific procedures developed to prevent foreseeing problems.
• Run in hole with BHA (bit + DP)
• Slowly trip in hole
• Closely watch drag
• Do not turn the pump on because of soft formation which can be easily side track
• Once tripping in hole to the previous TD, stage pump up slowly and watch for return. The well has a potential to collapse and pack off.
• If the pack off is seen, stop pump and work down string with rotation to free the pack off. Do not apply pressure if the pack off is occurred.
• Attempt to establish circulation to full drilling rate and closely watch returns
• Slowly work pipe and rotation while circulating
• Circulate with sea water and check pick up / slack off weight to ensure hole is clean
• Pump hi-vis sweep and displace well with water base mud
• Circulate hole clean and monitor weight
• Once the well is clean, trip out of hole to change BHA
• Make up BHA (mud motor)
• Trip in hole to casing shoe
• Orient tool phase to the original direction
• Trip in hole without rotation and pump to TD
With these procedures, we were able to get back to bottom, minimize risk and safely resume drilling operation.
We wish this lesson learned would help you in the future if you see the similar issue.